1. Field of the Invention
The present invention relates to an electronic blackboard system which is capable of processing as electric signals various types of information such as written information, erased information and designated information on a writing surface using a writing instrument, an eraser, and a designating rod and of outputting necessary information to an output unit.
2. Description of the Prior Art
As a conventional apparatus of this type, the present applicant already proposed one disclosed in Japanese Patent Application No. 2736/1986 "Electronic Blackboard Apparatus" (refer to Japanese Patent Laid-Open No. 160299/1987). A brief description thereof will be given.
A blackboard body comprises a tablet and a casing which is made of a non-magnetic material and whose surface is formed as a writing surface which can be used repeatedly. The tablet is arranged such that a multiplicity of position detecting members each mainly composed of an elongated magnetostrictive transmission medium element made of an amorphous alloy or the like, a first coil wound around one end thereof, and a second coil wound around the substantially entire length thereof are arranged in parallel in the X- and Y-directions.
If a pulse current is applied to the first coil, fluctuations in the magnetic field occur around the first coil due to the action of electromagnetic conversion, with the result that magnetostrictive vibratory waves are produced in the magnetostrictive transmission medium element. The magnetostrictive waves propagate through the magnetostrictive transmission medium element at a peculiar speed, and, in the mean time, fluctuations in the magnetic field corresponding to an electrical/mechanical coupling coefficient at each position.
To depict an image, if a magnetism generator for producing a steady magnetic bias, such as a marker or an eraser each having a magnet, is operated, the magnetic bias is applied to the magnetostrictive medium element, and the electrical mechanical coupling coefficient at that position is increased. For this reason, when the magnetostrictive vibratory waves reach the position immediately below the marker or the eraser, large fluctuations occur in the magnetic field, and a large induced voltage occurs in the second coil.
The time required from application of a pulse current to the first coil until the generation of a large induced voltage in the second coil is the time required as the magnetostrictive vibratory waves propagate from the position at which the first coil is wound around the magnetostrictive medium element to the position immediately below the marker or the eraser, and is proportional to the distance therebetween. Hence, if this time is measured, the coordinates of the marker or the eraser are detected.
If a comparison is made between the function of the marker and that of an eraser, the marker forms an image on the writing surface, while the eraser erases an image on the writing surface. Since their functions are utterly different, in order to obtain an image data corresponding to an image used for the marker and that for the eraser, it is necessary to accurately discriminate the coordinates for the marker and those for the eraser.
In the above-described apparatus, infrared-ray signals respectively having different frequencies are transmitted from the marker and the eraser when they are used, and these signals are received by the blackboard body. Thus, discrimination is made on the basis of the frequency as to whether the marker is being used or the eraser is being used.
With the above-described arrangement, however, it is necessary to provide the marker and the eraser with infrared ray-emitting elements, oscillation modulation circuits, switches, and batteries for operating them. Hence, there has been drawbacks in that the arrangement becomes complicated, large is size and weight, and that replacement or charging of the batteries must be carried out frequently, thereby aggravating the operating efficiency of the marker and the eraser.
In addition, the coordinates obtained as described above are normally stored in a predetermined image memory as image data corresponding to the image formed on the writing surface by the writing instrument. However, the image data is output from a printer as a hard copy, as necessary, or displaced on a display device connected to the apparatus, or transmitted to another similar electronic blackboard apparatus installed in a remote place via a communication line so as to be displayed on a display device thereof.
When a lecture is given by using an electronic board, there are cases where it is desirable to proceed with the lecture by indicating a desired position on the image on the writing surface with a finger or a rod-shaped instrument.
However, in the case of the above-described electronic board apparatus, there has been a drawback in that a person who is looking at the writing surface of the blackboard body can recognize the indicated position, but the indicated position cannot be known to the person who is viewing the display device.